It has been well known, generally that in case the layers of the stratified films of thermoplastic resin are stretched crosswisely, one in the vertical, the other in lateral direction, the tensile strength can be increased in all directions so that this film having a thickness one half that of single-layer film can be employed for making a strong bag for packing the same weight of heavy materials.
The bag made of polyethylene of high density, generally employed for packing heavy materials of about 25 kg. weight, has a thickness of 0.2 mm. To manufacture such a bag from a cylindrical film of plastic resin, such an annular die having a double extruding slits for molten resin has been used, or to make multi-layer cylindrical film such an annular die having an extruding slit has been used, introducing another kind of molten resin from a guide groove adjacent the slits to be laminated, or for the same purpose as above, two flattened films having stretched in the same direction, have been stuck crosswisely with each other. To form a strong bag by these films, it is necessary to perform several troublesome steps such as sticking together two flattened cylindrical films with their lengthwise directions mutually perpendicular with adhesive paste; folding back the cut film along the lengthwise center line; sticking together the edges opposite the lengthwise center line so as to form a tube; and closing one end of the tube to make a bag.
Those troublesome steps inflict much loss upon the advantage obtained by saving the material, decreasing the thickness so that the thick flattened film has been compelled to be used for the bag for packing heavy materials. Thus, an economical method and apparatus for manufacturing a multi-layer film is earnestly demanded by those in the same technical field as this art.